Biological Hierarchy

Nucleus

Stores DNA and controls gene expression.

Nucleolus

Assembles ribosomal subunits.

Ribosomes

Synthesize proteins from mRNA.

Rough ER

Folds and modifies proteins for secretion or membranes.

Smooth ER

Synthesizes lipids and detoxifies drugs.

Golgi apparatus

Modifies, sorts, and packages proteins.

Mitochondria

Generates ATP via oxidative phosphorylation.

Lysosomes

Degrade macromolecules and recycle cellular components.

Peroxisomes

Break down fatty acids and detoxify hydrogen peroxide (H{2}O{2}).

Cytoskeleton

Maintains cell shape and enables transport.

Centrioles

Organize microtubules during cell division.

Plasma membrane

Regulates transport and cell signaling.

Epithelial cells

Form protective and absorptive barriers.

Neurons

Transmit electrical and chemical signals.

Glial cells

Support, insulate, and protect neurons.

Skeletal muscle cells

Generate voluntary movement.

Cardiac muscle cells

Pump blood via rhythmic contraction.

Smooth muscle cells

Regulate involuntary organ movement.

Red blood cells

Transport oxygen using hemoglobin.

White blood cells

Defend the body against pathogens.

Fibroblasts

Produce extracellular matrix (ECM) and collagen.

Osteocytes

Maintain bone tissue.

Adipocytes

Store energy as fat.

Epithelial tissue

Covers surfaces and enables absorption and secretion.

Connective tissue

Supports, binds, and protects body structures.

Muscle tissue

Produces force and movement.

Nervous tissue

Detects stimuli and transmits signals.

Brain

Integrates sensory input and controls body functions.

Heart

Pumps blood to deliver oxygen and nutrients.

Lungs

Exchange oxygen and carbon dioxide (CO_{2}).

Liver

Metabolizes nutrients and detoxifies blood.

Kidneys

Filter blood and regulate fluid balance.

Stomach

Mechanically and chemically digests food.

Small intestine

Absorbs nutrients.

Large intestine

Absorbs water and forms feces.

Pancreas

Regulates blood glucose and digestion.

Skin

Protects the body and regulates temperature.

Integumentary system

Protects the body and prevents water loss.

Skeletal system

Supports the body and enables movement.

Muscular system

Produces movement and heat.

Nervous system

Coordinates rapid responses to stimuli.

Endocrine system

Regulates long-term body processes via hormones.

Cardiovascular system

Transports gases, nutrients, and wastes.

Lymphatic/Immune system

Returns fluid to circulation and defends against infection.

Respiratory system

Enables gas exchange for cellular respiration.

Digestive system

Breaks down food and absorbs nutrients.

Urinary system

Eliminates wastes and regulates blood chemistry.

Reproductive system

Produces gametes and enables reproduction.